Mr. Friess said this week he isn't likely to help Mr. Santorum ahead of Tuesday's Florida primary, because it's "not realistic" that the former Pennsylvania senator can prevail in the winner-take-all contest.

But Mr. Friess, 71 years old, said he plans to fund television advertisements for Mr. Santorum in other states in February and March. That could help keep Mr. Santorum competitive against his better-funded rivals and is one reason the fight for the Republican nomination will likely run at least through Super Tuesday on March 6.

Mr. Santorum's campaign has spent less than $1 million on television ads, according to Kantar Media, a company that tracks ad spending, while the primary independent group supporting him, the Red, White and Blue Fund, has spent more than double that amount. Mr. Friess is the largest donor to the fund; he wouldn't disclose the exact amount he has given.

Santorum had a great night in Jacksonville, but it remains unclear -- as it always has -- whether there's a financial path forward for his campaign. If there is one, Friess will probably be a significant part of it.